RNA interference (RNAi) by double-stranded (ds) small interfering RNAs (siRNA) suppresses gene expression by inducing the degradation of mRNAs bearing complementary sequences (Fire, A. et al. Nature 1998 391:806-811; Hamilton, A. J. & Baulcombe, D. C. Science 1999 286:950-952). Transfection of synthetic siRNAs into eukaryotic cells to silence genes has become an indispensable tool to investigate gene function, and siRNA-based therapy is being developed to knockdown genes implicated in disease (Elbashir, S. M. et al. Nature 2001, 411:494-8; Caplen, N. J., et al. Proc Natl Acad Sci USA 2001 98:97427; Rettig, G. R. & Behlke, M. A. Mol Ther 2012 20:483-512). More efficient ways to produce siRNAs are desired.